There may be another way to do this, possibly without using phpMyAdmin, but on the other hand, this might be the perfect opportunity for you to get somewhat familiar with phpMyAdmin, an absolutely necessary tool when dealing with database driven applications such as Coppermine.
That said, you could first try to repair the table using the "Repair Table" prompt. If that fails, you could try rebuilding the table manually. Note however, your exact cpgxx database name may differ. Just be sure to select your cpg15x database.
Databases -> cpg15_ -> cpg15x_usergroups -> and visually verify that the table access_level exists. If it exists, modify the fields to reflect the following changes (if applicable; leave empty fields empty):
Field: access_level
Type: tinyint(4)
Collation:
Attributes:
Null: No
Default: 3
Extra:
If the table does not exist, you will have to manually add it.
Click the Structure tab from the top navigation menu.
Below Print view towards the bottom of the page, you'll find Add __ field(s). Leave the number 1 in the textarea and select the radio button After.
Using the pull-down menu immediately to the right of After, select priv_upl_need_approval and click Go.
Use the above outline to add the field values to the table and save your changes.
Return to your gallery and verify that this particular error is resolved. There may or may not be others, depending upon the extent of the corruption.